The present invention relates to a method and apparatus for improving the efficiency of internal combustion engines, and more specifically to a method and apparatus for improving the efficiency of spark-ignition internal combustion engines having lean and/or diluted fuel-to-air mixture ratios.
It has been known for some time that the fuel efficiency of a spark-ignited (SI) internal-combustion engine can be improved by reducing the fuel-to-air mixture ratio from a stoichiometric value to a fuel lean value. SI engines having a fuel lean mixture are generally referred to as lean-burn engines. The fuel efficiency benefit of lean burn engines is well known, and described in Internal Combustion Engine Fundamentals, John B. Heywood, pg. 182, McGraw-Hill Book Company, 1988. It has also been known for some time that production of pollutants, including oxides of nitrogen (NOx), within the engine can be reduced by diluting the intake air with exhaust gas or employing significantly leaned fuel-to-air mixture ratios. The NOx reduction benefits of exhaust gas dilution and lean burn combustion are described in Automotive Fuel Economy, National Academy Press, pp. 217-219, 1992.
A problem with lean burn engines, however, is that of unstable combustion, partial burn, and misfire. More specifically, with increasing amounts of exhaust gas dilution and/or excess air, ignition quality deteriorates, and/or the mixture burns slowly and/or incompletely, resulting in high hydrocarbon (HC) emissions, significant variations in power output from cycle to cycle, and eventually misfire. The problem of poor combustion in lean burn engines is well known, and described in Automobile Technology of the Future, pp. 95-101, Society of Automotive Engineers, 1991. In port fuel injected prior art SI engines only a limited amount of mixture dilution can be employed without combustion quality becoming unacceptable. Consequently, the benefits of lean burn combustion and exhaust dilution are relatively small.
High levels of swirl within the combustion chamber can be used to achieve stable combustion of leaner fuel-to-air mixtures. However, the rapid swirling gas motion within the combustion chamber increases heat transfer and loss from the combustion charge, which substantially offsets the fuel economy benefit of lean burn combustion.
Significant research and development efforts are currently being directed towards spark-ignition direct-injection (SIDI) engines having ultra-lean-bum combustion. Mitsubishi Motor Corporation recently developed an SIDI engine that it now sells in Japan and Europe. The Mitsubishi SIDI engine is described in Society of Automotive Engineers (SAE) paper no. 970541, and in U.S. Pat. Nos. 5,740,777 and 5,806,482 issued to Hiromitsu Ando, General Manager of Engine Research Department at Mitsubishi et al. Mitsubishi claims that its SIDI engine has a low cost and a light load fuel economy improvement of 30% relative to conventional port fuel injection (PFI) engines. With SIDI engines, fuel is injected directly into the engine's cylinders to form a stratified charge having a rich or near stoichiometric mixture located near the spark plug at the moment of ignition. The rich or near stoichiometric mixture at the spark plug provide robust ignition and combustion of the fuel/air mixture within the cylinder. With SIDI and stratified charge, ultra-lean bulk-mixed fuel-to-air mixture ratios can be combusted yielding high fuel efficiency.
A problem with SIDI engines, however, is that with a stratified charge the fuel-to-air mixture ratio is rich or near stoichiometric at the spark plug, which results in high localized combustion temperatures and only marginal engine-out NOx reduction benefits. A much more sever problem with SIDI engines is that NOx in the exhaust gas can not be effectively reduced with known production-viable catalytic converters, and specifically when the SIDI engine is operating at a lean-burn high efficiency engine setting having free oxygen in the exhaust stream. Catalytic converter technology is evolving rapidly, however, if major developments in catalytic converter technology are not realized, SIDI engines may not be commercially viable in California and other states that phase in exceptionally stringent tail pipe emission standards.
Another approach that has been attempted for improving engine efficiency is variable compression ratio and adjustable valve control. Variable compression ratio systems with variable valve timing are described in U.S. Pat. No. 5,255,637 issued to M. M. Schechter, and in Automobile Technology of the Future, pp. 101-106, Society of Automotive Engineers, 1991. In prior art engines, variable compression ratio and variable valve control improves light load and part load engine efficiency, but provides only a minor improvement in peak engine efficiency. Variable valve control (also referred to as adjustable valve control) reduces light-load and part load pumping (e.g., throttling) losses, and variable compression ratio increases the light load and part load expansion ratio which improves engine efficiency. These engines have demonstrated a light load fuel economy improvement of approximately 12 to 15%. This is a small increase in fuel economy relative to the increased cost of the engine. A problem with variable compression ratio engines is that of high heat loss from the combustion chamber due to the relatively high chamber surface area to volume ratio, and poor combustion chamber shape. The problem of high heat loss is most sever in engines having high swirl levels and having a large valve overlap and subsequently large valve pockets in the pistons, which significantly increase combustion chamber surface area and heat loss. Additional improvements in light-load engine efficiency can theoretically be achieved by significantly reducing the displacement of the variable compression ratio engine, and employing supercharging to achieve maximum power requirements, however significant improvements in fuel economy have not been demonstrated with actual hardware, and these improvements do not improve peak engine efficiency. With respect to peak engine efficiency, prior art engines having variable compression ratio and variable valve control do not have significantly higher peak engine efficiency than conventional engines having a fixed compression ratio.